This invention generally relates to electronic systems and in particular it relates to analog amplifier circuits.
A typical prior art 2-stage amplifier block diagram is shown in FIG. 1. The circuit of FIG. 1 includes transconductors 20 and 22, resistors 24-26, capacitors 28-30, output resistance 32, output capacitance 34, input voltage Vin, and output voltage Vout. The prior art circuit of FIG. 2 shows the last section of a prior art folded cascode input stage coupled to an output stage as used in transconductors 20 and 22. The circuit of FIG. 2 includes output transistors 40 and 42, capacitors 44 and 46, quiescent current bias network 48, output voltage Vout, and transistors 50-53. To get maximum current drive, transistors 40 and 42 need to be very large devices. In order to not waste quiescent current, the gate-to-source voltage minus the threshold voltage (VGSxe2x88x92VT) of transistors 40 and 42 are set as small as possible. The maximum output current determines the size of transistors 40 and 42. The quiescent current is set by the second stage transconductance and the total harmonic distortion (THD) level of performance at small signal levels. At very low VGSxe2x88x92VT the quiescent current is less controlled as transistors 40 and 42 head for subthreshold region of operation. Another problem with very low VGSxe2x88x92VT is that the input stage cascodes may not have the head room required for maximum gain out of the stage. This is certainly true for modern CMOS processes where the VT of the devices are going down, and operating voltages for systems is going down as well. Another problem is the very large parasitic capacitance of the large output transistors 40 and 42. Typical load resistances for designs using CMOS power amplifiers are 32 Ohm all the way down to 8 ohm. With signal swings in the 4 volt range, this translates to current in the 70 mA to 250 mA range without sacrificing performance. These large power levels even with 90 Angstrom gate oxide on analog processes can result in PMOS devices sizes approaching 20,000 um (W/L). In these cases the parasitic capacitance of the output transistors 40 and 42 would be enormous, and would cause major problems with the stability of the amplifier. Using these prior art techniques, the quiescent current needed for proper operation of an amplifier of this type is very high. An amplifier of this type achieves good overall performance only at a maximum current to quiescent current ratio of 60-70. For proper operation the quiescent current level has to be such that the maximum load current is about 60-70 times the quiescent current. This does not solve the stability issue of the parasitic capacitance being very large.
A two-stage power amplifier includes: a first stage transconductor; and second stage having at least two parallel output branches supplying current to an output node, each output branch has an input coupled to an output of the first stage amplifier.